Method of making non-glare coated glass

ABSTRACT

A method for treating an alkali silicate non-glare coating with an acid in order to prevent alkali-induced haze is disclosed.

BACKGROUND

This invention relates generally to the art of non-glare glass and moreparticularly to alkali silicate coatings.

Some applications of glass, such as picture frames and CRT face plates,require a low-reflectance, i.e., non-glare, surface. A well-known andwidely practiced method for producing such a surface is by etching theglass surface.

U.S. Pat. No. 3,374,130 to Junge et al discloses an etching solution andprocess for producing a non-reflective surface on glass usinghydrofluoric acid.

As an alternative to acid etching the glass surface, U.S. Pat. No.3,326,715 to Twells discloses a method for producing non-glare, lowspecular reflectance films on glass articles by contacting the glasssurface with an aqueous solution of alkali silicate, and thereafterheating the glass at a sufficient temperature for a sufficient time todevelop a durable, adherent non-glare film on the glass surface.

U.S. Pat. No. 3,505,051 to Buckley et al discloses a continuous coatingprocess for forming a low gloss alkali silicate coating on a glassribbon wherein the glass ribbon surface is contacted, where thetemperature is preferably about 300° to 400° F., with an aqueous alkalisilicate solution.

SUMMARY

The present invention provides a method for improving the opticalproperties of non-glare alkali silicate coatings on glass by preventingthe development of haze in such coatings. The method of the presentinvention involves treating the alkali silicate non-glare coating with acompound, such as an acid or salt of a weak acid, which removes excessalkali from the coating. The acid treatment method of the presentinvention appears to render the non-glare alkali silicate coatingharder, more durable and initially clearer, as well as less susceptibleto the formation of haze.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides an inexpensive, rapid and safe method forproducing a non-glare surface on glass. Non-glare glass produced inaccordance with the present invention has even lower specularreflectance than chemically etched glass, and comparable resolution. Inaddition, non-glare glass produced in accordance with the presentinvention has both physical and chemical durability with respect tocleaning and environmental exposure. Non-glare glass produced inaccordance with the present invention is particularly suitable for usein picture frames as well as CRT monitor and television face plates.

Non-glare glass in accordance with the present invention is produced byfirst contacting a glass surface with a solution of alkali metalsilicate. The alkali metal silicate may comprise sodium silicate,potassium silicate or mixtures thereof. The ratio of alkali to silicatemay be varied over a wide range. The alkali silicate is preferably inaqueous solution, the concentration of which is preferably at leastabout 5 percent by weight. Suitable solutions are prepared by dilutingcommercially available alkali silicates with an appropriate amount ofwater. Preferred alkali silicates include KASIL potassium silicatesolutions available from Philadelphia Quartz and sodium silicatesavailable in various grades from Diamond Shamrock.

The glass surface may be contacted with the alkali silicate solutionwhile both are at ambient temperature. Alternatively, the glass may beat an elevated temperature. In a preferred embodiment of the presentinvention, a continuous ribbon of float glass may be provided with anon-glare surface by contacting the glass surface before it has cooledto ambient temperature with a solution of alkali silicate. The latentheat of the glass ribbon effectively evaporates the water and sets thealkali silicate coating.

While the alkali silicate solution may be applied to the glass surfaceby any convenient method such as dipping, brushing or roll coating, apreferred method for applying the alkali silicate solution to the glasssurface is spraying. U.S. Pat. No. 3,505,051 to Buckley, the disclosureof which is incorporated herein by reference, describes a low-glossalkali silicate coating process for glass which is readily adaptable tothe present invention. After the alkali silicate coating has beenapplied to the glass surface, it is necessary to treat the coatedsurface in accordance with the present invention in order to prevent"bloom".

It has been observed that untreated alkali silicate coatings develop ahazy appearance within about 24 hours. This hazy appearance has beenreferred to as "bloom", and is undesirable because it reduces thetransparency of the non-glare coating. The formation of "bloom" isbelieved to result from the reaction of excess alkali metal in thecoating with carbon dioxide in the air to form alkali metal carbonateswhich cause the hazy appearance. Once "bloom" occurs, it is difficult toremove and undesirable not to remove. Therefore, the prevention of"bloom" is the desired object of the present invention.

In accordance with the present invention, the alkali silicate coating ispreferably treated, as soon as practical after it is formed and beforeany "bloom" appears, with an acid or salt of a weak acid in order toeliminate the excess alkali which causes "bloom". A preferred method fortreating the alkali silicate coating is to rinse the freshly formedcoating with a solution of acid. A dilute aqueous solution of acid iseffective to eliminate alkali induced "bloom". Preferred acids includeinorganic acids such as nitric, phosphoric, boric, sulfuric andhydrochloric acids. Organic acids, such as acetic acid, may also beused, but may require higher concentrations to be effective. Salts ofweak acids, such as sodium bicarbonate, may also be employed. Nitricacid is particularly preferred in accordance with the present invention.Nitric acid is effective in sufficiently dilute solution that it may beadded to a conventional glass washer, thus eliminating a separate stepof applying the acid to the alkali silicate coating.

While acid treatment in accordance with the present invention iseffective at ambient temperature, higher temperatures may be employed tocompensate for lower concentrations of acid or shorter treatment cycles.Likewise, higher concentrations of acid may be employed with shortertreatment times or lower temperatures, or longer treatment times withlower temperatures or concentrations of acid. The three variables ofacid concentration, temperature and time of contact with the alkalisilicate coating may be varied over wide ranges and still result ineffective elimination of alkali induced "bloom". For example, oneparticularly preferred acid treatment sequence involves contacting thealkali silicate coating with 7 percent nitric acid at ambienttemperature for about 2 minutes, while another involves contacting thenon-glare coating with 3.5 percent nitric acid at 110° F. (about 43° C.)for about 30 seconds.

Heat treatment of potassium silicate non-glare coatings as taught inU.S. Pat. No. 3,326,715 to Twells, the disclosure of which isincorporated herein by reference, may fortuitously prevent "bloom" ofcertain alkali silicate compositions. However, such heat treatment maynot be practical for flat glass applications such as picture frameglass. Acid treatment in accordance with the present invention iseffective to prevent "bloom" in both sodium and potassium silicatecoatings, may be performed at ambient temperature, and is practicableon-line in a continuous process of coating a flat glass ribbon, as wellas treating alkali-silicate non-glare coatings on bent substrates.

The present invention will be further understood from the descriptionsof specific examples which follow.

EXAMPLE I

Non-glare alkali silicate coatings may be applied to individual glassplates and acid treated to prevent "bloom" as follows. In this example,a solution of sodium silicate is prepared by diluting one part by volumeof a commercially available sodium silicate solution with 19 parts byvolume of water. A suitable source of sodium silicate is DiamondShamrock Grade 40, which comprises one part Na₂ O to 3.22 parts SiO₂. Asample of 1/8 inch (3 millimeters) thick clear float glass is heated at500° F. (260° C.) for 5 minutes. A surface of the heated glass plate isthen sprayed with the sodium silicate solution for 10 seconds. The airpressure to the spray gun and distance from the glass surface areadjusted so that the water in the solution evaporates on contact withthe hot glass surface. After the coated glass plate has cooled, half ofthe sample is dipped for five minutes into an acid bath comprising onepart by volume concentrated (70 percent) nitric acid and 19 parts water.Before the acid treatment, the non-glare coated surface has a luminoustransmittance of 59 percent, a luminous reflectance of 5.3 percent and agloss measurement of 23 as measured by a Gardner glossmeter. After theacid treatment, the coated surface has a luminous transmittance of 64percent, luminous reflectance of 5.1 percent and a gloss measurement of26. The entire coated sample, both acid treated and untreated halves, isplaced in boiling water for 15 minutes. The acid treated half maintainsa clear appearance with a luminous transmittance of 66 percent, luminousreflectance of 5.1 percent and gloss of 26, while the untreated halfbecomes hazy and grayish with a luminous transmittance of 68 percent,luminous reflectance of 6.0 percent and gloss of 34. In normal exposureto the atmosphere at ambient temperature, the untreated half will"bloom" within 24 hours, while the treated half remains clear after 7days, which indicates that it will remain free from "bloom"indefinitely.

EXAMPLE II

An alkali silicate composition is applied continuously to a surface of afloat glass ribbon to form a non-glare coating which is acid treated toprevent "bloom" as follows. A solution of potassium silicate is preparedby diluting 45 parts by volume of KASIL No. 6 with 200 parts by volumeof water. KASIL No. 6, available from Philadelphia Quartz, is an aqueoussolution comprising 12.5 percent K₂ O and 26.3 percent SiO₂. The alkalisilicate solution is applied to the top surface of a float glass ribbonby means of spray guns reciprocating perpendicular to the direction ofmovement of the ribbon. In this example, clear glass with a thickness of6 millimeters and traveling at a line speed of 200 inches (about 5meters) per minute is contacted with the alkali silicate at a positionon the line where the glass surface temperature is approximately 400° F.(204° C.). The air pressure to the spray guns and distance from theglass surface are adjusted so that the water in the solution evaporateson contact with the hot glass surface. The alkali silicate coating setsas the glass cools and moves on to a cutting station where it is cutinto useful rectangular sizes. The individual coated glass plates arethen immediately acid treated by contacting them for about 15 secondswith a solution of 10 percent nitric acid. The acid-treated alkalisilicate coated non-glare glass products remain free from "bloom"indefinitely. In contrast, glass samples identically coated but not acidtreated "bloom", i.e., develop a white haze, in less than 24 hours.

EXAMPLE III

An alkali silicate coating is applied as in the previous example exceptthat the substrate is a ribbon of glass nominally 3 millimeters thick,traveling at a line speed of 370 inches (9.4 meters) a minute, and thesurface temperature is approximately 390° F. (199° C.) at the pointwhere the coating composition is applied to the glass surface. Thepressure in the fluid tank holding the alkali silicate solution is 5pounds per square inch, the atomizing air pressure is 50 pounds persquare inch and the spray guns are positioned approximately 10 inches(25.4 centimeters) from the glass surface. Following application of thealkali silicate coating, the glass was cut into useful sizes and treatedwith 10 percent nitric acid for 15 seconds as in the previous example.Again, acid-treated alkali silicate coatings remain free from "bloom"indefinitely, while untreated coatings "bloom" within 24 hours.

EXAMPLE IV

Individual glass plates are heated at 500° F. (260° C.) for 3 minutesand sprayed as in Example I with a solution comprising 62 grams of KASILNo. 6 potassium silicate solution in 100 milliliters of water to form apotassium silicate non-glare coating. Samples treated with 7 percentnitric acid for 30 seconds at ambient temperature do not develop"bloom".

EXAMPLE V

Glass plates are coated with a potassium silicate non-glare coating asin the previous example. Samples treated with 3.5 percent nitric acidfor 1 minute at 110° F. (about 43° C.) do not develop "bloom".

EXAMPLE VI

Glass plates with a potassium silicate non-glare coating as in ExampleIV treated with 3.5 percent nitric acid for 1 minute at ambienttemperature do not develop "bloom".

EXAMPLE VII

Glass plates with a potassium silicate non-glare coating as in ExampleIV treated with 3.5 percent nitric acid for 30 second at 110° F. (about43° C.) do not develop "bloom".

EXAMPLE VIII

Glass plates with a potassium silicate non-glare coating as in ExampleIV treated with 3.5 percent nitric acid for 30 seconds at ambienttemperature do not develop "bloom".

EXAMPLE IX

Glass plates with a potassium silicate non-glare coating as in ExampleIV treated with 3.5 percent nitric acid for 15 seconds at 110° F. (about43° C.) do not develop "bloom". This treatment simulates addition ofnitric acid to a conventional glass washer.

EXAMPLE X

Individual glass samples are heated to 500° F. (260° C.) for 5 minutesand contacted with a sodium silicate solution to form a non-glarecoating as in Example I. Samples treated with 5 percent boric acidsolution at room temperature for 5 minutes do not develop "bloom".

EXAMPLE XI

Glass samples coated with sodium silicate as in Example X and treated atambient temperature for 5 minutes with 5 percent hydrochloric acidsolution do not develop "bloom".

The above examples are offered to illustrate the present invention.Other acids which may be used to prevent bloom include mineral acidssuch as boric, phosphoric, sulfuric and hydrochloric, as well as organicacids such as acetic. In addition, aqueous solutions of salts of weakacids such as sodium bicarbonate, may be used to prevent "bloom". Thecomposition of the acid, as well as the concentration, time andtemperature of contact and so on sufficient to prevent "bloom" arevariables which may be easily determined by the practitioner. The scopeof the present invention is defined by the following claims.

I claim:
 1. A method for making a non-glare glass article comprising thesteps of:a. contacting a surface of the glass with an alkali silicatecomposition which forms an alkali silicate coating on said glasssurface; and b. contacting said alkali silicate coating with a compoundwhich removes excess alkali from the coating, said compound selectedfrom the group consisting of acids and salts of weak acids.
 2. A methodaccording to claim 1, wherein the step of contacting the glass surfacewith an alkali silicate composition is performed on a continuous glassribbon.
 3. A method according to claim 1, wherein the glass surface iscontacted with an aqueous solution comprising an alkali silicateselected from the group consisting of sodium silicate, potassiumsilicate and mixtures thereof.
 4. A method according to claim 1, whereinthe alkali silicate coating is contacted with an acid in solution.
 5. Amethod according to claim 4, wherein the coating is contacted with anaqueous solution of an inorganic acid.
 6. A method according to claim 5,wherein the coating is contacted with a dilute aqueous solution of amineral acid selected from the group consisting of nitric, phosphoric,boric, sulfuric and hydrochloric acids.
 7. A method according to claim6, wherein the coating is contacted with a dilute aqueous solution ofnitric acid.
 8. A method according to claim 7, wherein the step ofcontacting the alkali silicate coating with a dilute aqueous solution ofnitric acid is accomplished by adding nitric acid to a conventionalglass washing system.
 9. A method according to claim 7, wherein theconcentration of nitric acid is about 7 percent coating is contactedwith the acid solution at ambient temperature for about 2 minutes.
 10. Amethod according to claim 7, wherein the concentration of nitric acid isabout 3.5 percent and the coating is contacted with the acid solution ata temperature of about 110° F. for about 30 seconds.
 11. A methodaccording to claim 4, wherein the coating is contacted with an aqueoussolution of an organic acid.
 12. A method according to claim 11, whereinthe organic acid is acetic acid.
 13. A method according to claim 1,wherein the coating is contacted with an aqueous solution of a salt of aweak acid.
 14. A method according to claim 13, wherein the salt of aweak acid is sodium bicarbonate.